1. Field of the Invention
The present invention relates to an engine for a motorcycle. In particular, the present invention relates to an engine for a motorcycle having a crankcase, a cylinder block, and a cylinder head. The cylinder block extends upwardly and forwardly of the motor vehicle from an upper portion of the front end of the crankcase. The cylinder head is joined to the upper end of the cylinder block. The crankcase houses and supports a crankshaft, a transmission input shaft, and a transmission output shaft, which are arranged to have their axes oriented transversely of the motor vehicle. The transmission input shaft is offset upwardly with respect to a plane, which includes the axes of the crankshaft and the transmission output shaft.
2. Description of Background Art
Conventional engines for motorcycles generally have a cylinder block arranged to have an axis extending across the axis of the crankshaft (see, for example, Japanese Patent Laid-open No. Hei 11-29085).
In such engines for motorcycles, offsetting a transmission input shaft upwardly with respect to a plane which includes the axes of a crankshaft and a transmission output shaft is effective to reduce the interaxial distance between the crankshaft and the transmission output shaft to shorten the overall length of the engine in the longitudinal direction of the motorcycle for making the engine compact. However, the conventional engines fail to provide a sufficient upward offset of the transmission input shaft because of the cylinder block, which is positioned in the way. One solution would be to reduce the angle through which the cylinder block rises from the horizontal plane in order to provide a sufficient upward offset of the transmission input shaft. However, the front end of the cylinder head would be shifted forwardly. Accordingly, there would be an increase in the overall length of the engine in the longitudinal direction of the motorcycle. This increase would cancel out the reduction in the overall length of the engine in the longitudinal direction of the motorcycle, which is provided by the increased upward offset of the transmission input shaft.
The present invention has been made in view of the above drawbacks. It is an object of the present invention to provide an engine for motorcycles which has a transmission input shaft of which the upward offset is sufficiently increased without being obstructed by the cylinder block to effectively reduce the overall length of the engine in the longitudinal direction of the motorcycle.
To achieve the above object, there is provided in accordance with a first feature of the present invention an engine for a motor vehicle having a crankcase, a cylinder block extending upwardly and forwardly of the motor vehicle from an upper portion of the front end of the crankcase, and a cylinder head joined to the upper end of the cylinder block. The crankcase houses and supports a crankshaft, a transmission input shaft, and a transmission output shaft, arranged to have their axes oriented transversely of the motor vehicle. The transmission input shaft is offset upwardly with respect to a plane, which includes the axes of the crankshaft and the transmission output shaft. The cylinder block is disposed to have an axis passing forwardly of the axis of the crankshaft.
According to the above first feature, the cylinder block is disposed to have an axis passing forwardly of the axis of the crankshaft. Accordingly, if the angle through which the axis of the cylinder block rises from the horizontal plane is increased, then a free upper area of the crankcase, which is not obstructed by the cylinder block, is widened without changing the position of the front end of the engine. Therefore, the upward offset of the input shaft with respect to the horizontal plane can be sufficiently increased without being obstructed by the cylinder block. Therefore, the interaxial distance between the crankshaft and the output shaft is effectively shortened. As a result, the size of the crankcase in the longitudinal direction of the motorcycle is made compact, thus reducing the overall length of the engine in the longitudinal direction of the motorcycle for thereby effectively making the engine compact.
The offset of the axis of the cylinder block with respect to the axis of the crankshaft causes a reduction in the angle of inclination of the connecting rod under a maximum pressure in the expansion stroke of the engine. As a result, a side thrust which the piston receives from the inner surface of the cylinder bore is reduced, reducing the frictional resistance to the sliding motion of the piston, which contributes to lower fuel consumption.
According to a second feature of the present invention, in addition to the first feature, the cylinder block is disposed substantially parallel to a downward portion of a down tube of a vehicle frame.
According to the second feature, the dead space between the down tube and the cylinder head is minimized to reduce the wheelbase of the motorcycle.
According to a third feature of the present invention, in addition to the first and second features, a breather chamber communicates with the interior of the crankcase and is mounted on a portion of the crankcase near a rear surface of the cylinder block.
According to the third feature, the dead space present between the cylinder block and the input shaft in the crankcase is effectively utilized to form the breather chamber, further making the engine compact.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.